Haye is the biggest fight for either men (money-wise).
Vitali has stated on numerous occasions that his only desire's left at HW are to either unify against Valuev, or face Haye as it will attract big numbers for a HW fight in years.
I'm sure Wlad is also wanting a fight with Haye, although he has other options. Vitali in my opinion would give Haye the tougher fight of the two and be the most likely to win.